A heat exchanger is normally formed of a plurality of tubes oriented generally parallel to one another. In normal operation, a fluid to be heated or cooled is delivered through the inside of the tubes of such a heat exchanger. The outside surface of the tubes are contacted with a fluid which adds heat or removes heat as required. The plurality of generally parallel tubes forms a bundle. A set of end plates, known as heads, commonly support the bundle of tubes at each end.
Heat exchangers usually operate in a continuous fashion, often for months at a time. However, such continuous operation may be periodically interrupted to clean the tubes. The cleaning process is necessary to remove residue which collects on the inside surface of the tubes which reduces their heat transfer capability. The tubes are normally formed of metal which has a relatively high thermal conductivity. The material which may coat the interior of the tubes, however, has a much lower thermal efficiency for heat transfer. Therefore, the coating formed on the interior of the tubes is detrimental to the efficiency of the operation of the heat exchanger.
As residue builds up on the inside surface of a heat exchanger tube, the tube becomes less and less efficient. One way to counteract this effect is to raise the temperature differential across the tube. However, there are limits to this solution. For instance, the metal used in the tubes of the heat exchanger has a limited capacity for heat as a result of metallurgical considerations. Exceeding the design temperature differential across through the tubes increases fatigue and therefore reduces the useful lifetime of the heat exchanger.
In a well known U-tube design, the bundle of tubes takes a 180° bend or elbow at more or less the mid-point of the respective tubes. Fluid enters an inlet box which is separated by a divider plate from an outlet box. The fluid then flows through the head, through the tubes in first one direction then the reverse direction, back through the head and finally into the outlet box on the other side of the divider plate. Cleaning the tubes involves removing the accumulated coating material on the inside of the tubes and the difficulty of cleaning the inside surfaces of the tubes is exacerbated by the bend in the tubes. Also, as exchanger designs have improved, the effective length of the tubes has increased. This makes the task of cleaning the tubes more difficult because the long and relatively narrow tubes do not permit easy access to the tubes.
One way that the tubes can be cleaned is by pumping water or perhaps chemically active solvents into the tubes. That is successful but it has limitations. Moreover, since a typical heat exchanger includes a large number of tubes, it is necessary to undertake the cleaning in a repetitive fashion so that a large number of tubes can be cleaned.
In my U.S. Pat. No. 5,423,917, I described a system and a method for cleaning heat exchanger tubes. The system described therein has proven very successful. However, the system includes a control panel with a ganged set of valves to set up a shock wave to be injected into a tube. For particularly stubborn and tenacious fowling, especially involving hundreds of tubes, this manual alignment of the control panel valves can become tiresome and tedious. In related application titled Pump Valve Mechanism, I describe a solution to this problem. The cleaning action of that system, however, can be enhance by creating vapor bubbles in the stream of fluid. The cavitation creates a high intensity vibration, particularly at the harmonics of the region to be cleaned, thereby enhancing the cleaning operation of the system.